Fascinating thread.
I don't have any direct Gene Cunningham stories to tell. However, I will pass on a couple that Keith Johnson shared during the charity stream Special When Lit ran a few months ago as they were pretty entertaining and I've not seen them mentioned in this thread.
The first is that Gene originally was looking at buying Williams pinball and keeping it going. Initially this had a lot of interest with the engineers who had been working at Williams. Over time as the discussions continued the group shrunk down (as folks found other jobs and such) to Gene plus four others (one of which was Keith) that were committed to making it happen. It all was looking good until the equity split. The four non-Gene individuals all went in assuming equal equity stakes (so 20% each). At a subsequent meeting Gene stated he was getting 50%, the four other partners would have 49%, and Gene's daughter would get 1%. The potential partners insisted it needed to be equal and Gene said no. Keith stayed at Stern, the other three formed Pat Lawlor Design, and Gene did what Gene did.
The second involves Cactus Canyon. Years after the first story another company was looking to remake Medieval Madness and Cactus Canyon. Gene owned the rights to the patent on the Cactus Canyon drop targets (which were a single-coil design unique to that game). Gene wasn't paying the upkeep on it but still would have been able to keep it. But Keith had interest in the coil design for a possible game in the future, and wasn't pleased with Gene's prior behavior, and never told him and the patent expired.
Here's a link to my source. Keith tells the story far better than my summary. There's a brief gap between story one and story two. Link should take you to the very start of story one and story two ends about ten minutes after that.